Progressive miniaturization of chemical and biological instrumentation over recent years has spawned a new generation of systems for testing fluids. These new systems, known as microfluidic systems, are greatly simplifying and economizing chemical and biological testing. This miniaturization has led to the development of the “lab-on-a-chip” that is highly conducive to automated mass production at very low costs, much like an electrical integrated circuit chip. The size of the lab-on-a-chip also opens up new applications that were either unavailable or too unwieldy to implement with typical laboratory equipment.
The driving force behind this revolution is the microfluidics. A basic building block of a microfluidics system is the fluid valve, such as the valve 10 depicted in FIG. 1. In one embodiment the valve is made a three-layer structure, which includes a fluid layer 12, a membrane 14, and a gas layer 16. When the valve is in a closed state shown in FIG. 1A, pressurized gas is vented to a gas channel 18 and which pushes the membrane 14 against the seat surface 21 of a valve seat 20 on the fluid layer 12. When the valve is in an open state the membrane 18′ is pulled down by a vacuum applied in the gas channel 18 so that the seat surface 21 is clear of the membrane, as shown in FIG. 1B. Thus, in the open state the membrane 14′ is retracted from the valve seat 20 permitting fluid to flow through the fluid microchannel 22.
In certain microfluidic applications multiple fluids are mixed, such as in the exemplary system depicted in FIG. 2. The system includes a number of valves V1-V9 that control the flow of the fluids, in this case Fluid A and Fluid B. Pumps P1 and P2 are provided to draw the fluids from their respective reservoirs and propel them through the Mixer. In a typical microfluidic system the pumps are peristaltic pumps. The valves and pumps are controlled in a particular sequence to introduce quantities, or “slugs” Slug A and Slug B, of the different fluids to the Mixer. With the fluids in the Mixer, the valves V5 and V9 are closed and the fluids are continuously looped by the pump P2 until they are appropriately mixed. At that point, the valve V8 is closed and V9 opened to discharge the mixed fluids.
However, one problem that arises is that air bubbles become trapped in micro-volumes of fluids. The air bubbles can be particularly disruptive when fluids are mixed, as in the system shown in FIG. 2. In some cases, mixing efficiency can be greatly diminished or even prevented. Where the air bubbles are particularly large the pump P2 may not function properly so that the fluid micro-volumes stall within the Mixer.
Due to the insidious nature of trapped air bubbles in microfluidic applications, purge valves have been developed. One such purge valve devices is a latch valve developed by Urbanski et al. See J. P. Urbanski, W. Thies, C. Rhodes, S. Amarasinghe, and T. Thorsen; “Digital Microfluidics Using Soft Lithography”, Lab Chip 6: 96-104 (2006). This latch-type purge valve relies upon partially deflecting the valve membrane, such as membrane 14 shown in FIG. 1A, to block the flow of an emulsified aqueous sample, for instance, while allowing the immiscible oil phase to pass. Surface tension of the fluid slug prevents its passage through the reduced valve opening. However, this type of valve requires more complex operations and fabrication limitations, which make it hard to prevail in the microfluidic applications. For instance, the purge latch is actuated by a pressure source independent from the control lines used to control the closed-open state of the valve. Alternatively, use of a common pressure source requires more sophisticated control of the gas pressure provided in the gas line 18.
Moreover, this type of purge valve device is limited to pressure-operated valves. For instance, the partially deflecting or partially opening attribute cannot be readily replicated in an electrostatic valve. In an electrostatically actuated device there is no intermediate stage of attraction between the electrodes—there is only on and off.
There remains a need for a microfluidic system with the capability of purging air bubbles within a micro-volume of fluid that is low-cost, simple to operate, and easy to fabricate. There is also a need for a purging system that can be used universally with all microfluidic components.